1. Field of the Invention
The invention relates to diodes having a dynamic negative resistance over a predetermined frequency range. More particularly, the invention relates to the class of dynamic negative resistance diodes known as IMPATT. More particularly still, the invention relates to a special type of IMPATT diode termed the Read diode.
2. Description of the Prior Art
The original structure used for Read diodes is shown in U.S. Pat. Nos. 2,899,646 and 2,899,652 both of Aug. 11, 1959, and both issued to W. T. Read, Jr. Such diodes basically comprise a semiconductor junction for producing an avalanche of charge carriers, a drift region of intrinsic or nearly intrinsic semiconductor material, and a highly doped buffer region for extracting charge carriers. A dynamic negative resistance is produced by fixing the length of the drift region such that charge carriers produced by the avalanche are delayed in their transit through the drift region such that they are recovered at the buffer zone 180.degree. out of phase with the voltage across the diode but in-phase with the current flow through the diode.
Standard IMPATT diodes, on the other hand, were constructed using only three semiconductor layers, these diodes comprising a linear arrangement of three layers consisting of a layer of semiconductor material of a first conductivity type adjacent a center layer of the opposite conductivity type. A buffer zone of the same second conductivity type as the middle zone but of a much higher doping density provided a buffer zone for extraction of charged carriers, avalanching and drift both took place in the center region. As the efficiency of such diodes is related to the length of the drift region as compared with the length of the avalanche region and as both avalanching and drift took place within the same portion of semiconductor material of a uniform doping density, the efficiency of such diodes tended to be rather low. Read diodes promised to have a much higher efficiency than the original IMPATT diodes as the avalanching region was confined within the somewhat narrow portion of semiconductor material specifically provided as an avalanche zone.
However, Read-type diodes have heretobefore not been commercially successful for a number of practical reasons. Foremost among these reasons was the inability to provide a sufficiently thin avalanching region to make the efficiency of the Read diode greater than or even equal to the efficiency of the previously developed IMPATT diodes. Also, nonuniformities in the length of the avalanching region greatly affected the frequency response of the diodes causing the yield per processed wafer to be too low for practical production. A third and little recognized problem was that in the original Read-type diodes the electric field produced in the drift region necessary for causing the avalanche carriers to proceed through the drift region was made to extend up to and to be non-zero at the junction between the drift region and buffer zone. This presence of an electric field at this juncture caused the generation of minority carriers. The minority carriers were swept back into the drift region by the electric field through to the avalanche region. Here the minority carriers caused generation of further majority carriers by impact ionization within the avalanche zone. The majority carriers thusly generated were then swept back into the drift region out of phase with the carriers already produced having the proper phase relationship with external signals. The addition of this second group of majority carriers to the current within the drift region further lowered the efficiency of the device.
Furthermore, the relationship between the depth of the avalanche region and the efficiency of a Read-type diode device was imperfectly understood. No procedure existed for designing a practical device with proper avalanche region depth and proper doping levels so as to give a desired operating efficiency at a predetermined frequency.